How can I do a chemical conjugation with PEG-COO- with NH2-aptamer? Whether any linker that can be dissociated in acidic environment can be used (because of cancer environment)? Please do the needful.
Thank you.
For conjugation, any active ester chemistry (e.g. NHS ester which are commercially available) should do, resulting in an amide. For release, you could design your construct in a way that it will be hydrolyzed by a specific protease or peptidase.
You might use a different linker that also has an ester function (or utilizes an ester link instead of the amide) so it may be cleaved by esterases. Depends all on the stability of your construct in the body, before it has reached the tumor site, and the intended increased susceptibility to degradation within the tumor. Not an easy way to go, IMHO, it requires a lot of variation, optimization and screening.
Wolfgang Schechinger Thank you for the reply.
Right now I'm opting for PEG-COOH and NH2 conjugation with EDC and NHS technique. Will that bond cleave particularly at tumor environment? Or should i opt for some other linker?
Thanks
You will get an amide bond. Easy to perform on the bench. But it won't hydrolyze just because a probably little more acidic environment in the tumor. The easiest way to go might be finding/synthesizing a modified PEG linker that maybe contains an ester bond, sth like PEG-(C=O)O-(CH2)n-COOH or design the peptide in a different way (Eg.,. use Pep-COOH, activate it (you probably need to discuss your options with the people who are synthesizing your peptide) and make an ester with PEG-OH). But extracellular esterases might be faster than you wish and inactivate your molecule.
Another possibility could be -SS- based linkers (proposed example structure: PEG-SS-(CH2)n-COOH) which are quite stable under extracellular conditions (oxidizing), but will be reduced to thiols, once inside a cell.
Not sure what is commercially available, you might have to team up with chemists.
Actually, this PEG-COOH and NH2 conjugation will be done in aqueous media, that will be further used to form a emulsion system, in that case, will the drug get released in acidic environment? Because right now I'm having only COOH-PEG-COOH so it's a bit difficult for me to purchase ester linked or S-S linked peg.
Amid bonds are quite stable. To hydrolyze them, you need to boil them for hours in a sealed tube with pretty concentrated hydrochloric acid (amino acid analysis of proteins, 30 years ago) if you don't have an enzyme that can do the job. There is a reason why amide bonds are found in proteins.